[Bldg-sim] Minimum OA Ratio for VAV System

[Doebber, Ian]

ASHRAE 90.1-2004 does not specifically state anywhere a MAXIMUM OA RATIO
that a VAV system can not exceed.  Yet ASHRAE 62-2004 insinuates this
with its reference to Ventilation Effectiveness based on OA Ratio (Zp)
in Table 6-3.  

It would make perfect sense that a VAV system should be limited to never
have an OA Ratio exceed say 0.45.  This is applicable especially in
heating where the effects of Thermal Stratification will cause
Ventilation Short Circuiting.

VAV control systems have the capability of limiting the OA Ratio which
has extremely large Energy Implications.  For example, if the MAXIMUM OA
RATIO is 0.45, then the VAV system is limited to the how low the Total
Supply Air can reduce to.  Consequently a lot more fan energy will be
required to maintain that higher Flow Rate.

Has anyone ever incorporated a MAXIMUM OA RATIO in their VAV System
Design?

Regards,
Ian Doebber